Per the present state of the art, for practical purposes, noninvasive blood pressure is measured in one of two primary ways. The first method is the ascultatory method wherein blood flow in the brachial artery is impeded via a pressure cuff and “listened to” for purposes of assessing the state of the flow or lack thereof and consequent pressures relating to cardiac systole and diastole.
The second primary method is the oscillometric method wherein a pressure cuff is applied and the variation of pressure oscillations in the cuff are used to determine systolic and diastolic pressure. Due to limitations with each method, the ascultatory and oscillometric methods are sometimes used in combination to maximize accuracy.
Even so, per the present state of the art, neither the ascultatory method, nor the oscillometric method, nor their use in combination is adequately complete for purposes of assessing the comprehensive status of arterial pressure and patient health. This is because neither adequately address characterization of the arterial pressure wave resulting, or not, from respiratory sinus arrhythmia. By and large, both auscultatory and oscillometric assessment methods assume that the heartbeat rate varies little, where, in reality, the heart beat rate and resultant background arterial pressure can and should vary widely. This should-be wide variation is due to the phenomenon of respiratory sinus arrhythmia (RSA), this being the fact that the heartbeat tends to increase coincident with inhalation and decrease coincident with exhalation. Respiratory sinus arrhythmia gives rise to the phenomenon known as the “arterial pressure wave” which rises and falls with respiration. The arterial pressure wave is a well recognized physiological phenomenon, “With each cycle of respiration, the arterial pressure usually rises and falls 4 to 6 mm Hg in a wavelike manner, giving rise to so-called respiratory waves in the arterial pressure. During deep respiration, the blood pressure can rise and fall as much as 20 mm Hg with each respiratory cycle.” (Medical Physiology—Guyton and Hall, 2000) Because, diastolic pressure is highly related to respiratory sinus arrhythmia, under ideal breathing circumstances, this author (Elliott) has measured systolic and diastolic pressures that differ by as much as 70 mm Hg. and diastolic pressures less than 50 mmHg, diastolic pressure being particularly affected by robust respiratory sinus arrhythmia. Fundamentally, the arterial pressure wave is a consequence of increasing heartbeat rate and heart output coincident with inhalation and a decreasing heartbeat rate and output coincident with exhalation.
When a human subject is breathing in a relatively rapid and shallow manner, as do most people when in the state of rest or semi-activity, their heartbeat varies only slightly between its upper and lower limits, for example 81-86 beats per minute as depicted in FIG. 1. Relatively rapid shallow breathing results in a minimal arterial pressure wave and for this reason a relatively small variation in pulse pressure as assessed by oscillometric measurement as depicted in FIG. 2. Under these circumstances, present ascultatory and oscillometric methods yield approximately the same measurement result.
In the case where breathing is properly slow and deep, the heartbeat rate tends to vary to a much wider degree, for example 60-96 beats per minute as depicted in FIG. 3. A widely varying heart rate variability results in a widely varying arterial pressure wave which results in a widely varying arterial pressure. Because the arterial pressure wave modulates pulse pressure, it can result in widely varying systolic and diastolic pressures, diastolic pressure being particularly affected. This case is depicted in FIG. 4. Under these circumstances present oscillometrically based assessment methods can yield widely varying results and systolic/diastolic readings that are significantly inaccurate relative to the ascultatory method. Nor can auscultatory methods easily take arterial pressure wave implications into account because there is no knowledge of breathing frequency, depth, or phase relative to measurement timing.
Present auscultatory and oscillometric techniques share the common shortcoming of not presenting the total picture of arterial pressure. In prior patent Ser. No. 10/932,636, it is asserted and explained that suboptimal breathing is in fact a root cause of hypertension, the reason being that rapid breathing while at rest results in autonomic nervous system acceleration resulting in an increased heartbeat rate, decreased heart rate variability, and increased heart duty cycle. This relationship between breathing and heart rate is depicted in FIG. 5. Yet, the medical community has yet to draw an overt connection between breathing and blood pressure. In part, this is due to the lack of attention paid to respiratory sinus arrhythmia and the resultant “arterial pressure wave” and its relevance to acute and chronic systemic blood pressure. These shortcomings have resulted in the perpetuation of a narrow and erroneous understanding in the medical community as to the root cause of hypertension.
Also, owing to the aforementioned oscillometric assessment limitation, a person that is breathing at the proper rhythm and depth cannot employ present oscillometric measurement devices with confidence, a given device yielding significantly different readings with each measurement as well as different manufacturers yielding significantly different results. This is not because of limitations of oscillometric technology but because the unwritten objective of manufacturers of oscillometric measurement units is consistency between auscultatory and oscillometric assessment of “systolic” and “diastolic” pressures as well as the general lack of recognition of the importance of arterial pressure wave assessment within the industry.
The present state of the art definition of “arterial pressure wave” is the difference between “systolic” and “diastolic” blood pressures. Here again, a lack of recognition exists that that arterial pressure wave varies greatly as a function of respiratory sinus arrhythmia and that the important metric is the amplitude of the arterial pressure wave as a function of breathing.
The foregoing makes clear the fundamental issues and limitations of present auscultatory and oscillometric arterial pressure assessment methods. The present invention addresses these shortcomings by facilitating assessment of arterial pressure taking into account the respiratory sinus arrhythmia induced arterial pressure wave under varying breathing conditions.